Gradually Varied Flow - Normal Depth US HW?

[rlondeen]

Flow Regime: Sub-critical & open surface flow

In a scenario where one needs to do backwater calculations for a channel / culvert in subcritical flow, is assuming normal depth as the US water level acceptable? At some point the backwater calculations will converge to normal depth, but I've never read anywhere where it is acceptable to use the normal depth. I assume this is because it may not converge by the time you hit your next analysis point?

 

[beej67]

If you already know it's subcritical regime, the upstream boundary condition shouldn't matter. You should be making assumptions for the downstream boundary condition, and then calculating the depths upstream based on that downstream assumption. Does that make sense?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[LincolnPE]

No. Upstream boundary condition is only used to analyze supercritical flow.

Refer to Open-Channel Hydraulics. Ven Te Chow. McGraw-Hill, New York, 1959

 

[rlondeen]

Thanks beej67. I was trying to avoid doing backwater calculations completely (setting up a spreadsheet without using macros) by just assuming that the normal depth of the culvert/channel upstream location was the HW if it is "close enough" to the actual depth... However, I have recently realized that since culverts are so hydraulically short, it will often not be close enough to assume normal depth at the upstream side since there isn't enough length to normalize back to normal depth.